1. Field of the Invention
The present invention relates to a thin film magnetic head including a magneto-resistive effect element having a CPP structure for reading magnetic field strengths of a magnetic recording medium or the like as signals. Also, the invention relates to a head gimbal assembly and a magnetic disk device including the thin film magnetic head.
2. Description of the Related Art
As the surface recording density of a magnetic disk device has lately been advanced, there is a need for enhancing the performance of a thin film magnetic head. As a thin film magnetic head, a complex type thin film magnetic head has widely been used that has a structure of a reproducing head having a read-only magneto-resistive (MR) effect element (hereinafter also referred to as an MR element) and a recording head having a write-only magnetism conversion element layered on a substrate.
MR elements include an anisotropic magneto-resistive (AMR) element using the AMR effect, a giant magneto-resistive (GMR) element using the GMR effect and a tunnel magneto-resistive (TMR) element using the TMR effect.
The characteristics of a reproducing head should particularly be high sensitivity as well as high output. As a reproducing head that satisfies such requirements, a GMR head using a spin-valve type GMR element has already been mass-produced.
Moreover, a GMR element having a structure that allows a sense current to flow in the perpendicular direction (layering direction) relative to the surface of each layer configuring the GMR element, that is, a current perpendicular to plane (CPP) structure (a CPP-MR element) has been developed as a next generation element. A TMR element that has already been mass-produced and put into practical use falls under the category of CPP-MR elements as well.
An MR element is placed between upper and lower shield layers formed of soft magnetic metal films and is disposed in a manner of being sandwiched between insulating layers referred to as gap layers. The recording density of bit direction is defined by a shield gap (also known as read gap length) between the upper and lower shield layers. As the recording density has lately increased, there is an increased need for a narrow shield gap or narrow track for the reproducing element of a reproducing head.
In the thin film magnetic head including such a magneto-resistive effect element, a bias magnetic field application layer is disposed on each of two sides of an MR element. These bias magnetic application layers allow application of a so-called vertical bias to a free layer configuring part of the element. As a result, the generation of noise can be suppressed, thus allowing the operation of detecting a predetermined external magnetic field.
Therefore, the magnetic characteristics of the bias magnetic field application layers (e.g., retentivity, residual magnetic field and squareness ratio) are very important for securing the characteristics and reliability of the MR head. If residual magnetization is small, a magnetic field applied to the free layer for magnetic field response control becomes insufficient, which causes the variation of the symmetry of reproduced waveforms as well as Barkhausen noise. If retentivity is small, a bias magnetic field tends to be reduced under influences of a magnetic field applied from shield layers disposed on the upper and lower portions of an element or influences from a recording medium, resulting in the variation of the symmetry of reproduced waveforms as well as Barkhausen noise like the case as described above. These types of problems are particularly noteworthy where the element has a structure in which a gap between the shield layers disposed on the upper and lower portions of the element is extremely small (i.e., a narrow shield gap).
In order to form bias magnetic field application layers, insulating layers are usually provided first on both side parts of an MR element. Subsequently, foundation layers are formed on the insulating layers and then bias magnetic field application layers on the foundation layers (Japanese laid-open application numbers 2008-97700 and 2008-84446).
Japanese laid-open application number 2008-97700, which is the prior art, discloses the use of SiO2, AlN, Si3N4, SiN, etc. for insulating layers as well as Cr, CrTi, CrMo, CrV, W, TiW, etc. for foundation layers.
Moreover, Japanese laid-open application number 2008-84446, which is also the prior art, discloses alumina for insulating layers as well as the use of Cr or CrTi for foundation layers.
Nevertheless, there is some possibility that the magnetic characteristics of the bias magnetic field application layers (e.g., retentivity, residual magnetic field and squareness ratio) will significantly change by the design specification of the layer configuration of insulating layers and foundation layers even though magnetic materials may remain the same. Accordingly, there remains a need to further improve the specification of the insulating layers and the foundation layers.
Under such circumstances, the present invention was made. The object of the present invention is to propose a novel thin film magnetic head incorporating a new design specification for the layer configuration of insulating layers and foundation layers relating to bias magnetic field application layers, which decrease the rate of occurrence of noise. Moreover, it is to propose a novel thin film magnetic head incorporating a new design specification for the layer configuration of insulating layers and foundation layers relating to bias magnetic field application layers, which is superior in thermal stability after annealing treatment.